Efficiency in electro-chemical processes, such as electrolysis, electroplating, electrowinning, electro-organic synthesis, and waste recovery, depends to a substantial extent upon the surface area of the electrode. Electrodes have been constructed with ridges or convolutions to increase the surface area. Sandblasting also has been used to roughen the electrode surface, and thus provide a larger surface area. These known techniques have been found to have limited effectiveness in increasing the surface area.
More recently carbon fibers for electrodes which provide large surface areas have been described in U.S. Pat. Nos. 4,046,663, 4,108,754 and 4,108,757. The electrodes comprise a plurality of carbon fibers arranged generally parallel to one another and clamped at one end to an electrical connection. Although these electrodes may have large surface areas, they provide relatively poor electrical connections. Specifically, a large number of carbon fibers invariably break as a tow of such fibers is clamped into an electrical connection. This breakage of fibers adversely affects the electrical effectiveness of the tow. Additionally, the mechanical connection of carbon fibers results in an undesirably high electrical resistance at the connection. Consequently, the theoretical efficiencies of the electrodes are not attainable because of the mechanically destructive and inefficient electrical connections.
The electrodes shown in U.S. Pat. Nos. 4,046,663 4,046,664, 4,108,754 and 4,108,757 also act as a wick, causing the electrolyte to be drawn up into the area of the terminal. When the electrolyte evaporates, a salt residue remains which affects the electrical connection. The salt deposits thermally shield the terminal causing heat buildup, increased resistance, and eventually terminal failure by bridging. Even if wicking and fiber damage could be controlled, there would be poor electrical connection to the fibers in the center of the bundle.
Several attempts have been made to place metallic coatings on the carbon fibers so that tows of the plated carbon fibers can be used more efficiently as electrodes in various electro-chemical processes. In most instances, the plating applied to these carbon fibers has been discontinuous, brittle, and expensive to apply. For example, U.S. Pat. No. 4,132,828 shows the vacuum deposition of nickel onto carbon fibers. The coating taught by this patent, however, is not continuously in contact with the carbon fibers and will easily break and fall off if the fiber is twisted.
Electroless nickel baths also have been employed to plate carbon fibers. However, this plating process is slow, expensive to carry out, and again results in inferior discontinuous coatings. Another undesirable coated fiber is shown in U.S. Pat. No. 3,622,283.
In view of the above, it is an object of the present invention to provide fiber containing electrodes having large surface areas, efficient electrical connections, and continuous metal coatings on fibers with high bond strengths therebetween.
It is a further object of the subject invention to provide plated and unplated fiber electrodes which can be bent, wrapped, woven or knitted into a variety of configurations for efficient use in electro-chemical cells.
It is still another object of the invention to provide electro-chemical cells and processes with electrically conductive fibers constructed into electrodes without the drawbacks of the prior art electrodes.